The present invention relates to a non-aqueous electrolyte secondary battery (hereinafter, battery), and especially relates to batteries of which electrochemical properties such as the charge/discharge capacity and charge/discharge cycle life have been enhanced by improvements in negative electrode materials and non-aqueous electrolytes.
In recent years, lithium secondary batteries with non-aqueous electrolytes, which are used in such fields as mobile communications devices including portable information terminals and portable electronic devices, main power sources of portable electronic devices, domestic portable electricity storing devices, motor cycles using an electric motor as a driving source, electric cars and hybrid electric cars, have characteristics of a high electromotive force and a high energy density.
The lithium ion secondary batteries which contain an organic electrolytic solution, and use carbon materials as negative electrode active materials and lithium-containing composite oxides as positive electrode active materials, have higher voltage and energy density, and superior low temperature properties compared with secondary batteries using aqueous solutions. As these lithium ion batteries do not use lithium metal for the negative electrode, they are superior in terms of cycle stability and safety, thus are now being commercialized rapidly. Lithium polymer batteries using macromolecular (polymer) gel electrolytes which contain an organic electrolytic solution, have been also under development as a new thin and light batteries.
As for polymer electrolyte batteries, various researches have been conducted on the subject since Armand et al. disclosed a polymer electrolyte battery comprising polyethylene oxides and electrolytic salts (Second International Meeting on Solid Electrolytes, Extended Abstracts, p20-22, 1978, U.S. Pat. No. 4,303,748). Although various polymer electrolytic materials have been mentioned, for example, in the conductive polymer edited by Naoya Ogata, Kodansha, 1990, and Polymer Electrolyte Reviews, Vol. 1 and 2, Elsevier, London (1987, 1989), ionic conductivity of these polymer electrolytic materials at room temperature is only about 10xe2x88x924-10xe2x88x925 S/cm.
As another method to improve ionic conductivity, different types of electrolytes which can easily achieve the ionic conductivity of 10xe2x88x923 S/cm have been disclosed, for example, in J. Electrochem, Soc., 137, 1657 (1990), U.S. Pat. No. 5,085,952, U.S. Pat. No. 5,223,353 and U.S. Pat. No. 5,275,750. These electrolytes are called polymer gel electrolyte in which solvents of organic electrolytic solutions are added to polymers as plasticizers. Polymer batteries using these polymer gel electrolytes are expected to achieve the same performance as lithium ion batteries by improving ionic conductivity. However, in respect of capacity, both positive electrode and negative electrodes of these polymer batteries need to be made of composite materials containing polymers as well. Thus, the mass (density) of active materials in the casing of the battery is reduced. Therefore, when the same materials are used for both positive and negative electrodes, energy density of the lithium polymer secondary batteries becomes lower than that of the lithium ion batteries.
When a high-capacity lithium metal is used as a negative electrode material, dendritic deposits are formed on the negative electrode during charging. Over repeated charging and discharging, these dendritic deposits penetrate through separators and polymer gel electrolytes to the positive electrode side, causing an internal short circuit. The deposited dendrites have a large specific surface area, thus their reaction activity is high. Therefore, they react with plasticizers (solvents) of the polymer gel electrolytes, lowering charge/discharge efficiency. Due to these reasons, the lithium secondary batteries using lithium metal as a negative electrode material have a low reliability and a short cycle life.
To suppress the formation of such dendrites, it has been disclosed that lithium alloys such as lithium-aluminum alloy and a wood""s alloy are used instead of lithium metal. Metals capable of forming alloys with lithium and alloys containing at least one such metal can be used as a negative electrode material with a relatively high electrochemical capacity in the initial charge/discharge cycle. However, by repeatedly alloying with and de-intercalating lithium, they may undergo a phase change even when the crystal structure of the original skeletal alloy is maintained, or sometimes, the crystal structure of the skeletal alloy of elements changes. In this case, particles of the metal or alloy which are host materials of the lithium, an active material, swell and shrink. As the charge/discharge cycle proceeds, crystal grains are stressed and cracked, thus particles are pulverized and come off from the electrode plate. As the crystal grains are pulverized, resistance and contact resistance of the grain boundaries increase. As a result, resistance polarization during charging and discharging increases. Thus, when charging is conducted at a controlled voltage level, charging depth becomes shallow, limiting the amount of electricity charged in the battery. On the other hand, during discharging, the voltage drop occurs by the resistance polarization, reaching the discharge-termination voltage early. Thus, superior charge/discharge capacity and cycle properties can not be expected.
If currently used solvents such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, gamma-butyro lactone, and gamma-valero lactone are used in a system in which lithium metal or a lithium alloy is used in a negative electrode, the electrolyte may decompose and gas may be produced when the battery is fully charged and stored at high temperatures. Moreover, if the battery is repeatedly charged and discharged, parallel to the charge/discharge reaction of the negative electrode, the electrolyte is gasified, lowering the charge/discharge efficiency, resulting in decreased cycle properties.
Nowadays, lithium secondary batteries which use, as a negative electrode material, carbon materials capable of intercalating and de-intercalating lithium ions, are commercially available. In general, lithium metal does not deposit on carbon negative electrodes. Thus, short circuits are not caused by dendrite. However, the theoretical capacity of graphite which is one of currently used carbon materials is 372 mAh/g, only one tenth of that of pure Li metal.
If graphite-group carbon materials are used as a negative electrode material, and propylene carbonate is adopted for an electrolytic solution, the electrolytic solution is decomposed at potentials more precious than that of lithium metal. Consequently, lithium ions are not intercalated between layers of graphite, and the battery does not function. Considering these points, currently commercialized lithium secondary batteries with the graphite-group used for negative electrode materials frequently use electrolytic solution containing ethylene carbonate. However, the melting point of ethylene carbonate is 37xc2x0 C. higher than room temperature. Therefore, at low temperatures, ionic conductivity of the electrolytic solution for lithium ions plummets, lowering charge/discharge priorities.
When inorganic compound materials such as TiS2 are used as a negative electrode active material, intercalation and de-intercalation of lithium occur at sufficiently more precious potentials compared with lithium metal and lithium alloys. Thus, even when the negative electrode active materials come in contact with the electrolytic solution, reductive decomposition does not occur. Moreover, even when propylene carbonate is used for the electrolytic solution, intercalation and de-intercalation are not impeded by decomposition as is the case with the graphite materials, therefore, a wider range of electrolytic solutions is applicable. However, potentials of the negative electrode using the foregoing inorganic compound materials is precious, causing voltage of the battery to inevitably become low. This is a disadvantage of achieving higher energy density.
Regarding the supporting electrolytes, the thermal stability of LiClO4, LiBF4, and LiPF6 needs to be improved. Furthermore, above fluorine-containing inorganic anion salts react with minute amounts of water contained in an electrolytic solution and decompose.
To form a totally solid-state battery by using, as non-aqueous electrolytes and lithium ion conductor glass-type solid electrolytes, solid electrolytes in powder form need to be mixed in the electrode to secure and maintain ionic conductivity in the electrode. However, electrodes constructed in the manner mentioned above are brittle, and incapable of absorbing expansion and shrinkage of the electrode materials that occur during charging and discharging. In other words, the electrode itself merely expands as it lacks resilience. Therefore, once it expands, it does not shrink. Therefore, some particles fail to contact with one another properly, increasing the number of particles in the electrode material which can not contribute to charging and discharging. As a result, the properties of the battery are lowered.
Other known negative electrode materials include pure metallic materials and pure non-metallic materials which form compounds with lithium. For example, composition formulae of compounds of tin (Sn), silicon (Si) and zinc (Zn) with the maximum amount of lithium are respectively Li22Sn5, Li22Si5, and LiZn. Within the range of these composition formulae, metallic lithium does not normally deposit. Thus, an internal short circuit is not caused by dendrite. Furthermore, the electrochemical capacities between these compounds and each element mentioned above are respectively 993 mAh/g, 4199 mAh/g and 410 mAh/g; all are larger than the theoretical capacity of graphite.
As other compound negative electrode materials, the Japanese Patent Laid-Open Publication No. H07-240201 discloses a non-metallic siliside comprising transition elements. The Japanese Patent Laid-Open Publication No. H09-63651 discloses negative electrode materials which are made of inter-metallic compounds comprising at least one of group 4B elements, P and Sb, and have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
However, the foregoing high-capacity negative electrode materials have the following problems.
Negative electrode materials of pure metallic materials and pure non-metallic materials which form compounds with lithium have inferior charge/discharge cycle properties compared with carbon negative electrode materials. The reason for this is assumed to be the destruction of the negative electrode materials caused by their volume expansion and shrinkage.
On the other hand, as negative electrode materials with an improved cycle life property unlike the foregoing pure materials, the Japanese Patent Laid-Open Publication No. H07-240201 and the Japanese Patent Laid-Open Publication No. H09-63651 respectively disclose non-metallic silisides composed of transition elements and inter-metallic compounds composed of at least one of group 4B elements, P and Sb, and which have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
Batteries using the negative electrode materials of the non-metallic silisides composed of transition elements are disclosed in the Japanese Patent Laid-Open Publication No. H07-240201. The capacities of the embodiments of the invention and a comparative example at the first cycle, the fiftieth cycle and the hundredth cycle suggest that the batteries of the invention have improved charge/discharge cycle properties compared with lithium metal negative electrode materials. However, when compared with a natural graphite negative electrode material, the increase in the capacity of the battery is only about 12%.
The materials disclosed in the Japanese Patent Laid-Open Publication No. H09-63651 have a better charge/discharge cycle property than a Lixe2x80x94Pb alloy negative electrode material according to an embodiment and a comparative example. The materials also have a larger capacity compared with a graphite negative electrode material. However, the discharge capacity decreases significantly up to the 10-20th charge/discharge cycles. Even when Mg2Sn, which is considered to be better than any of the other materials, is used, the discharge capacity decreases to approximately 70% of the initial capacity after about the 20th cycle. Thus, their charge/discharge properties are inferior.
The present invention relates to a non-aqueous electrolyte secondary battery comprising:
a positive electrode and a negative electrode capable of intercalating and de-intercalating lithium, and
as non-aqueous electrolytes,
lithium ion conductive glass-type solid electrolytes, or
polymer gel electrolytes using a polymer comprising at least one of;
polyalkylene oxides or their derivatives, specific polymers which containing fluorine, polyacrylonitrile, polyester, and copolymers of methacrylate and ethylene oxides.
The negative electrode is characterized by its main material which uses composite particles constructed in such a manner that at least part of the surrounding surface of nuclear particles containing at least one of tin, silicon and zinc as a constituent element, is coated with a solid solution or an inter-metallic compound composed of an element included in the nuclear particles and at least one element (exclusive of the elements included in the nuclear particles) selected from a group comprising group 2 elements, transition elements, group 13 elements and group 14 elements (exclusive of carbon) of the Periodic Table. With the foregoing construction, an internal short circuit between the positive electrode and the negative electrode caused by the expansion of the negative electrode materials can be restricted, thereby achieving high capacity, high energy density batteries with superior charge/discharge cycle properties.
The lithium ion conductive glass-type solid electrolytes of the present invention includes; at least lithium sulfide as a first component; one or more compounds selected from silicon sulfides, phosphor sulfides and boron sulfides as a second component; and one or more compounds selected from lithium phosphate, lithium sulfate, lithium borate, lithium silicate as a third component. The lithium ion conductive glass-type solid electrolytes is synthesized with above three components.
The construction of the present invention achieves novel non-aqueous electrolytic secondary batteries which rarely suffer generation of gas when stored at high temperatures. Moreover, even when the batteries are repeatedly charged and discharged, charge/discharge efficiency of their negative electrode does not decrease. The batteries can be used in a wide range of temperatures. Furthermore, the batteries enjoy high energy density and a lower reduction rate of discharge capacity when used repeatedly as well as high charge/discharge properties.